SUMMARY OF WORK It is well known that the sympathetic and parasympathetic nervous system produce opposing effect on cardiac function. In cardiac ventricular myocytes, sympathetic effects which are mainly mediated via b adrenergic receptor (bAR)-Gs-adenylyl cyclase (AC) pathway are generally stimulatory while parasympathetic stimulation antagonizes these effects by M2-muscarinic receptor-Gi-AC pathway. Recent studies from this lab have found that: although both b1AR and b2AR are functionally present in canine ventricular myocytes, in contrast to b1AR stimulation, b2AR agonists zinterol induced positive inotropic effect is not accompanied by the increases in cAMP, PKA activity, or cAMP linked protein phosphorylation. Thus, the focus of this study is to assess if a M2 receptor agonist, carbachol(cch), can still reverse the effects of b2AR stimulation in canine ventricular myocyte; and if so, what is the underlying mechanisms? The results showed that although cch virtually had no effects on the basal contractility or non-receptor-induced positive inotropic effect by increasing bath [Ca 2+] from 1 mM to 5 mM, it completely abolished both the positive inotropic effect and the relaxation effect elicited by zinterol via a PTX-sensitive pathway. More importantly, the equipotent contractile response of a cAMP analogue, CPT-cAMP, which bypass the AC and directly activate PKA, was also largely inhibited by cch in a PTX sensitive manner, suggesting that the downstream of PKA is also involved in the inhibitory action of cch in canine cardiomyocyte. Since the equipotent contractile response of a phosphatase inhibitor, calyculin A, was not suppressed by cch, and calyculin A pretreatment at threshold concentration totally abolished the antagonism of cch on CPT-cAMP mediated positive inotropic effect, it is very likely that the inhibitory effect of cch is, at least in part, mediated by an activation of phosphatase. Furthermore, while calyculin A pretreatment did not prevent the antagonism of cch on zinterol's effect, it markedly prolonged the time course of the cch's antiadrenergic effect, indicating that phosphatase is also involved in the cch's antagonism on b2AR stimulation. In conclusion, a PTX-sensitive-phosphatase signaling is involved in the inhibitory effect of M2 receptor activation, which may contribute to the antagonism of cch on the effects of b2AR stimulation in canine ventricular myocyte.